Analysis of long-lived particle decays with the MATHUSLA detector

The MATHUSLA detector is a simple large-volume tracking detector to be located on the surface above one of the general-purpose experiments at the Large Hadron Collider. This detector was proposed in [J. P. Chou, D. Curtin, and H. J. Lubatti, Phys. Lett. B 767, 29 (2017)] to detect exotic, neutral, long-lived particles that might be produced in high-energy proton-proton collisions. In this paper, we consider the use of the limited information that MATHULSA would provide on the decay products of the long-lived particle. For the case in which the long-lived particle is pair-produced in Higgs boson decays, we show that it is possible to measure the mass of this particle and determine the dominant decay mode with less than 100 observed events. We discuss the ability of MATHUSLA to distinguish the production mode of the long-lived particle and to determine its mass and spin in more general cases.

Figure 1

Exclusion reach of MATHUSLA, corresponding to four expected decays in the detector (solid curves), compared to the best-case ATLAS projection (dotted curves), for pair production of LLPs in exotic Higgs decays $h\to XX$, from [43]. The three curves of each set correspond to three different values of the LLP mass. Sensitivity up to the big bang nucleosynthesis lifetime limit [45] is possible.

Figure 2

We assume the MATHUSLA detector geometry shown on the left. On the right, we schematically show the patterns of charged tracks reconstructed for different final states of LLP decay.

Figure 3

Kinematics of the two-body decay of an LLP: The left-hand figure shows the angles ${\theta }_1$, ${\theta }_2$; the right-hand figure illustrates the boost back to the LLP rest frame in which the two products are back to back. Note that ${\widehat{p}}_i$ or ${\widehat{p}}_i({\beta }_X)$ denote momentum vectors normalized to unit length in each frame. For convenience we work in the coordinate system where ${\widehat{p}}_X$ is along the $z$-axis and the decay products are in the $(x,z)$ plane (far right).

Figure 4

Distribution of LLP boost $b=|{\overrightarrow{p}}_X|/m$ for different LLP masses. The solid histograms show the truth-level value of $b$, which is also close to the distribution of reconstructed boosts for $X$ decay to two charged particles. The dotted histograms show the distribution of reconstructed boosts for hadronic LLP decays using the sphericity-based method of Sec. 3d with only upwards going tracks.

Figure 5

The expected number of LLP decays in MATHUSLA required to measure the LLP mass to relative precision $\mathrm{\Delta }m/m$ using only upwards-traveling tracks, for LLP decay to muons (left) and jets (right). $N_{\mathrm{obs}}$ refers to the number of decays in the detector volume regardless of whether the event is reconstructed. For muons, $N_{\text{reconstructed}}=\epsilon N_{\mathrm{obs}}$ where $\epsilon \approx 0.95(0.55)$ for $m_X=15(55)\mathrm{GeV}$. For hadrons, $\epsilon \approx 1$. The plots show the statistical uncertainty only.

Figure 6

Distribution of charged particle multiplicity (only counting upwards traveling tracks) for $m_X=15,55\mathrm{GeV}$ and gluon jets and for flavor-democratic (gauge-ordered) and heavy-flavor dominated (Yukawa-ordered) quark jets.

Figure 7

Truth-level boost distribution of a 35 GeV LLP produced in a variety of different production modes, from left to right: $Z\to XX$, $h\to XX$, vector boson fusion through a 200 GeV mediator in the $t$ channel, decay of gluinos with mass 800 and 1500 GeV, vector boson through a $WW\to XX$ contact interaction, and $q\overline{q}\to XX$ through a vector contact interaction.

Figure 8

Bias (left) and spread (right) of the sphericity-based boost measurement for average charged particle multiplicity $N_{\mathrm{ch}}=10$.

Figure 9

Bias of the sphericity-based boost measurement for average charged particle multiplicity $N_{\mathrm{ch}}=10$, showing the effect of speed measurement with precision $\delta v$ in the thermal pion case.